a). Field of the Invention
The present invention relates to inspection of semiconductor fabrication, in particular, to measurement of electron shading damage in plasma-assisted semiconductor fabrication.
In this specification, the terminology "electron shading damage" is used to mean the damage in semiconductor device caused by excess positive charge flown into a conductive surface due to shading of electrons.
b). Description of the Related Art
The tendency in the art of semiconductor integrated circuit devices is to achieve higher integration (smaller dimensions), and larger diameter of the semiconductor wafer. In that situation, low-pressure, high-density plasma is indispensable to micro-patterning of semiconductor devices. In such plasma-assisted semiconductor patterning, the plasma to be used is so designed that the positive charges and the negative charges are well balanced therein so as to protect the semiconductor substrates from being adversely influenced by the charges injected thereinto from the plasma.
However, in plasma-assisted processing of a layer covered with a resist (or insulating film) mask with apertures having a high aspect ratio, there occurs charging damage of so-called electron shading damage that is peculiar to high-density plasma-assisted process, even though plasma having uniform charge distribution is used.
It is believed that the electron shading damage is caused by the difference in behavior between electrons and ions. Suppose a case where a conductive layer to be etched is electrically isolated from a semiconductor substrate while opposing to the substrate via a thin insulating film such as a gate oxide film or the like.
In general, the semiconductor substrate and the plasma shall have a bias potential (electric field) therebetween, and the positively-charged ions among the plasma enter the substrate while being accelerated. Whereas, the negatively-charged electrons are decelerated in the electric field. As a result, the ions are incident to the substrate nearly vertically thereto, while the electrons are obliquely incident to the substrate since the velocity components in the directions parallel to the surface of the substrate become relatively large.
Where the conductive layer to be etched has an insulator pattern thereon that surrounds a conductive surface, the electrons proceeding to the conductive surface in oblique directions are shaded by the insulator pattern. Ions of normal incidence are not shaded by the insulator pattern and can be injected to the layer in the direction vertical to the surface of the layer. This results in an overflow of positive charges into the conductive surface.
The electrons having been thus captured on the side walls of the insulator pattern form an electric field which repulses the incoming electrons. The electrons having small kinetic energy in the vertical direction are substantially repulsed by the electric field. In contrast to this, however, the positively-charged ions are not repulsed but accelerated by the electric field formed by the captured electrons, and enter the conductive surface below the insulator pattern. This further augments the overflow of positive charges in the conductive surface. It is believed that the electron shading damage occurs in this way.
Positive charges shall accumulate in the conductive layer below the insulator pattern. Where the conductive layer is connected with an insulated gate electrode, an electric field will be imparted across the gate-insulating film by the accumulated positive charges. When a tunneling current passes through the gate-insulating film due to this electric field, the positive charges stored in the conductive layer will reach a stationary state. However, the gate-insulating film is degraded by the tunneling current.
When the gate-insulating film is thick, the tunneling current hardly passes through the gate-insulating film and the positive charges stored in the conductive layer increase. Then, the thus-accumulated positive charges will generate an electric field in which electrons are attracted to the surface of the conductive layer. When electrons are attracted to the surface of the conductive layer by this electric field, the charge stored in the conductive layer will reach a stationary state even when no tunneling current passes through the gate-insulating film.
In MOS transistors of smaller dimensions, the thickness of gate oxide films becomes smaller. For such thinner gate-insulating films, the tunneling current passes more easily. Thereby the life of the gate-insulating films becomes short by the tunneling current caused by electron shading.
In low-pressure, high-density plasma-process of a semiconductor substrate, measurement of the degree of electron shading damage (the charging damage caused by electron shading) is indispensable for improving the reliability of the semiconductor devices to be fabricated.
For the measurement of electron shading damage, for example, is known a method of connecting a comb-shaped antenna to the gate electrode of a MOS transistor followed by subjecting the comb-shaped antenna to plasma treatment to measure the threshold voltage shift in the MOS transistor induced by the plasma treatment.
When a tunneling current passes through the gate oxide film by the electron shading damage, the threshold voltage of the MOS transistor is shifted. Measuring the shifted threshold voltage makes it possible to calculate the amount of charges having passed through the gate oxide film.
This method requires preparing MOS transistors for electron shading damage measurement. For establishing a process, it is required to optimize various process parameters. In such a situation, preparing MOS transistor samples only for monitoring the process condition brings about increase in production costs.
For more simple samples for measurement, another method of using samples of simpler structure utilizes a MOS capacitor (with a gate electrode only) but not a MOS transistor structure. According to this method of using a MOS capacitor as a sample for measurement, however, the threshold voltage cannot be measured. In this method, therefore, the breakdown voltage of the insulating film in the MOS capacitor is measured. However, the accuracy in the breakdown voltage measurement in the MOS capacitor is low, and it is therefore difficult to quantify the degree of charging damage according to this method.
As described above, the process monitoring using test elements of MOS transistor enables the quantitative measurement of the degree of electron shading damage, but the cost for sample preparation is high. On the other hand, if aMOS capacitor is inspected as the test device, the cost for sample preparation can be reduced, but the measurement accuracy is low.
In general, a conductive layer as formed on the entire surface of a semiconductor substrate is electrically connected with the semiconductor substrate anywhere (for example, in the scribe lines). In this condition, any positive charges entering the conductive layer cause no problem so far as they directly flow away into the semiconductor substrate.
After the main etching of the conductive layer to be etched (to give an interconnection pattern) in an open space on a semiconductor substrate has finished, the remaining, non-etched interconnection pattern is electrically isolated from the semiconductor substrate. Then, over-etching should be done to etch residual conductive layer in narrow spaces. In this condition, electron shading damage is caused by the charges that enter the layer to be further etched owing to the micro-loading effect, and flow through the gate oxide film.
Therefore, the electron shading damage depends on the time from the finish of the main etching in the open space to the finish of the over-etching in the narrow space. In other words, measurement of the electron shading damage depends on the time for the over-etching of the layer to be etched, and therefore, if the over-etching time is short, the electron shading damage is difficult to be measured with accuracy.
For detailed studies of electron shading damage caused by plasma to be measured, it is desired to measure the electron shading damage for a desired, satisfactorily long period of time.